Multi-metal corrosion-resistant diffusion coatings

ABSTRACT

A method is provided herewith for the production of multi-metal diffusion coatings on metal articles providing prolonged protection against chemical or galvanic corrosion of the surface of the coated article during prolonged exposure to corrosive conditions, and particularly high saline content marine atmospheres, especially where the protective coating is also subjected to mechanically erosive and abrasive environments, with the multiplicity of coating metals being selected so that the combination thereof provides a coating varying through the thickness thereof from outer surface toward the interface of coating layer and coated article so that the mechanical resistance to chemical corrosion is greatest at the outer surface but decreases as the coating thickness is eroded away, while the components of the coating offering sacrificial or cathodic protection are more concentrated adjacent the coating-article interface so that cathodic protection of the coated article increases as the coating is removed by abrasion or erosion. As illustrative, the outer coating surface includes a high concentration of metallic components inherently resistant to saline corrosion and/or abrasion, although offering less cathodic protection for the coated article; while inner layers of the coating are rich in metallic components offering high sacrificial or cathodic protection, although less erosion or saline corrosion resistance.

United States Patent [191 Brill-Edwards I MULTI-METALCORROSION-RESISTANT DIFFUSION COATINGS [75] Inventor: HarryBrill-Edwards, San Antonio,

Tex.

[73] Assigneez. Chromallay American Corporation, West Nyack, NY.

22 Filed: July 28, 1971 21 Appl.No.: 167,030

Related US. Application Data [62] Division of Ser. No. 733,303, May 31,1968, Pat. No.

[52] US. Cl. 117/71 M, 29/1962, 29/1965, 29/1966, 117/130 R, 204/37 R, 204/147 [51] Int. Cl B44d H46 [58] Field of Search 204/148, 147, 35, 37R; 117/71, 109, 22, 130 R; 29/1962, 196.5,

[451 Apr. 30, 1974 v[ ABSTRACT A method is provided herewith for theproduction of multi-metal diffusion coatings on metal articles providingprolonged protection against chemical or galvanic corrosion of thesurface of the coated article during prolonged exposure to corrosiveconditions, and particularly high saline content marine atmospheres,especially where the protective coating is also subjected tomechanically erosive and abrasive environments, with the multiplicity ofcoating metals being selected so that the combination thereof provides acoating varying through the thickness thereof from outer surface towardthe interface of coating layer and coated article so that the mechanicalresistance to chemical corrosion is greatest at the outer surface butdecreases as the coating thickness is eroded away, while the componentsof the coating offering sacrificial or cathodic protection are moreconcentrated adjacent the coating-article interface so that cathodicprotection of the coated article increases as the coating is removed byabrasion or erosion. As illustrative, the outer coating surface includesa high concentration of metallic components inherently resistant tosaline corrosion and/or abrasion, although offering less cathodicprotection for the coated article; while inner layers of the coating arerich in metallic components offering high sacrificial or cathodicprotection, although less erosion or saline corrosion resistance.

5 Claims, No Drawings MULTI-METAL CORROSION-RESISTANT DIFFUSION COATINGSThis is a division of application Ser. No. 733,303, filed May 3l, 1968,now US. Pat. No. 3,642,457.

This invention relates to the protection of metal articles fromcorrosion in highly saline and/or marine atmospheres despite the factthat such articles are also to be used under circumstances where severemechanical erosion or abrasion of the protective coating is likely to beexperienced and, more particularly, to multicomponent diffusion coatingsin which the extent of sacrificial protection of the coated article fromcomponents in the coating increases as the thickness of the protectivecoating is eroded away by mechanical abrasion by utilizing for the outersurface of the coating components which'have marked erosion andcorrosion resistance, although little potential for cathodic protection,while utilizing in inner layers of the coating adjacent the coatedarticle components with a high potential for cathodic protectionalthough being of reduced erosion or corrosion resistance.

As purely illustrative of certain applications to which this inventionis particularly related,-and the problems incident thereto, one may notethe circumstances and environments incident to the operation of certaincomponents of jetaircraft engines in highly saline atmospheres (such aslow flying aircraft, particularly helicopters, adjacent seaports andseacoasts), particularly where the highly saline marine atmosphere alsoincludes substantial amounts of erosive particles of sand, coral dust,etc. Whereas the anti-corrosion problems of turbine components (ascompared to compressor components) of such jet aircraft enginessubjected primarily to only the impingement of extremely hightemperaturecombustion gases may relate so much to oxidation resistance at such hightemperatures that other possible sources of corrosion becomeinsignificant, the compressor components of such jet engines experiencequite different problems. For example, the compressor components, awhile rarely subjected tocoperating temperatures above 900 F, aresubjected to direct impingement of highly saline atmospheres at the airintake, which may also include substantial amounts of abrasiveparticulate material, in addition to being subjected. to tremendousmechanical stresses from centrifugal forces, thermal shock, vibrationetc., and under circumstances (particularly with singleengine aircraft)where premature failure of the compressor parts for whatever reason maybe catastrophic, regardless of the fact that the greater problem ofextremely high temperature oxidation-resistance of the hot turbinecomponents may have otherwise been solved.

Thus, if the particular materials (such as high strength ferrous alloyslike those designated in the Aerospace Material Specifications of theSociety of Automotice Engineers as AMS 5508, AMS 5616, AMS 6304, etc.)are utilized for their mechanical strength in the compressor, they mayhave inadequate saline resistance or erosion resistance, without someprotective surface treatment, and to an extent which cannot assure auseful and failure-free life comparable to that of the high temperaturerefractory metals or superalloys utilized in the hot turbine componentsof the same jet engines.

But jet engine compressor components must function and maintain theirfunctioning within very low tolerances and clearances which do not admitof a substantial build up of corrosion products (rust, pulverous oxides,etc.), and even miniscule pitting from saline corrosion (characteristicof virtually all stainless steel parts subjected to saline atmospheres)may reduce the mechanical strength factor many-fold and to an extentwhich is inimical with adequate performance in such highly stressedparts as compressor blades, compressor rotors, and, particularly, thejunctures therebetween subjected to great centrifugal and othervibrational and mechanical forces in a rotating manner and at speedswhich cannot tolerate substantial radial imbalances.

If it is attempted to protect against the saline corrosion to beexpected in such applications in marine atmospheres by, for example, adiffused nickel-cadmium electroplated protective coating (as set forthin AMS procedure 2416 D) or by the diffusion coating of metallic zincinto the surface of the ferrous structures to be protected, less thanoptimum results may be encountered. For example, considering anickel-cadmium coating in which the nickel is adjacent the ferroussubstrate (as is almost inevitable because cadmium is virtuallyinsoluble in iron and, thus, is difficult satisfactorily to diffuse intoa ferrous surface), neither component is sufficiently anodic relative tothe substrate to assure absolute sacrificial protection in local areaswhere the electroplated nickel has not adhered to the ferrous substrate;and, furthermore, when the surface layer of cadmium is removed orpenetrated by the inevitable erosion or abrasion encountered in suchspecific applications as noted above, the exposed sublayer of nickel iselectrochemically noble relative to the ferrous substrate so that salinegalvanic corrosion is actually intensified to an extent even greaterthan that which would occur in the same atmosphere with uncoated ferroussubstrates.

Similarly, relying on a protective diffusion coating of zinc mayeliminate or curtail the exaggeration of electrochemical salinecorrosion of the ferrous substrate, but zinc is itself highly corrosivein saline atmospheres producing voluminous quantities of white rustdeposite (such as complexes of zinc hydroxide and zinc chloride) whichbuild up on the surface of the parts and diminish the mechanicaltolerances and aerodynamic effectiveness of the surfaces thereof.

As will be understood, the foregoing difficulties are not necessarilydisastrous in various types of hardware, even for the aforementionedapplications. For example, the blades and shroud vanes of a jet enginecompressor may be considered much more susceptible to mechanical erosionor abrasion by air-home sand or coral dust particles than is thecompressor rotor disc around the periphery of which the blades arefastened. Yet all are subjected to whatever electrochemical corrosion isinherent in such saline environments, and it is believed unrealistic toattempt to design a chemically or electrochemically or cathodicallyprotective coating without recognizing that it may be subjected tomechanical erosion or abrasion onslaught at a rate which far exceeds theexpected life of the coating even if it be considered as whollyelectrochemically sacrificial. By the same token, if the protectivecoating be subjected both to mechanical erosion and sacrificial cathodicprotection, particularly under circumstances where the rate of erosionexceeds that of cathodic sacrifice, then the need for cathodicprotection increases the more the thickness of the coating diminishes byerosion at least to the extent that the less the coating mechanicallyprotects against erosion and corrosion (i.e., the thinner the coatingbecomes), the greater is the need for cathodic protection of thesubstrate.

In accordance with this invention, by contrast to previous suggestedsolutions of such problems, there is provided a multi-componentdiffusion coating for ferrous metal articles or substrates having acomposition gradient through the thickness of the coating from outersurface to coating-substrate interface varying in chemical andmechanical properties whereby, regardless of the resistance of thecoating layers to mechanical erosion or abrasive wear, the outer surfacehas concentrated therein components particularly resistant to salinecorrosion (although of less potential for cathodic protection of thesubstrate), while inner portions of the coating adjacent the substrateto be protected have concentrated therein components of increasedpotential for cathodic or sacrificial protection of the substrate(although perhaps less than maximumly resistant to either mechanicalerosion or direction chemical corrosion by the saline atmosphere). Yetall such components are applied in accordance herewith, and integratedwith the metal article substrate to be protected by diffusion techniquesproducing substantially complete integration of the various coatingcomponents with the substrate article to be protected in a manner whichsubstantially eliminates mechanical cracking or separation of thecoating relative to the substrate despite intense mechanical or thermalshock stresses which may be encountered in use. As a further feature ofthis invention, there are provided a plurality of techniques forapplying such a multi-component protective coating, either in one ormore stages of application, to provide segregated but metallurgicallyunified areas through the coating of varying compositions andspecifically located with regard to the nobility, sacrificialpotentials, cathodic protection characteristics, and corrosion anderosion resistances to achieve optimal mechanical and electrochemical orcathodic protection of the metal article being coated.

With the foregoing and additional objects in view, this invention willnow be described in more detail, and other objects and advantages willbe apparent from the following description and the appended claims.

In attempting to select and devise the appropriate combinations ofmulti-metallic systems for achieving adequate or optimum protectivecoatings in accordance herewith, a number of independently variable, andoften inconsistent, criteria need be kept in mind. For example, in orderto avoid mechanical failures or separation under thermal shock orphysical cracking of the coating materials, it is preferred that anactual diffusion of the coating material into the substrate be obtained(as compared, for example, to merely an electro-deposition or plating,although that may be an appropriate means for preliminary deposition ofthe coating metal provided it can be subsequently diffused into thesubstrate). Yet, there are somemetals which might provide desiredelectrochemical protection but which are not susceptible to actualdiffusion into a ferrous substrate (cadmium, for example, is virtuallyinsoluble in iron and, thus, not susceptible for solid state diffusionthereinto).

Similarly, some metals are available for good initial corrosionprotection or resistance to corrosion in saline atmospheres andadequately diffusable into iron, but

which do not render the desired cathodic or sacrificial protection ofiron in a saline atmosphere (nickel, for example, which, being morenoble than iron, actually increases the iron corrosion in any area wherethe nickel protective coating may have been worn away sufficiently toexpose the iron substrate surface). Other metals which may be readilydiffusable into iron and electrochemically appropriate for sacrificialprotection thereof may not be sufficiently initially resistant to salinecorrosion or mechanical erosion to last as a protective coating longenough for their sacrifical protection to be realized (zinc, forexample, is readily diffused into ferrous substrates and sacrificiallyprotective thereof, but so susceptible to initial saline corrosion as toproduce such large initial quantities of white rust as frequently tooutweigh the ultimate or subsequent protection which might be afforded).

Nevertheless, considering the foregoing criteria, and others as will benoted hereinafter, multi-component protective coatings have been devisedin accordance herewith to give satisfactory or enhanced protection underthe saline-corrosive and mechanically erosive environments noted byutilizing a combination of various metals to form a multi-layer coatingwhere all components are diffused or interdiffused with a ferroussubstrate and under circumstances where the composition gradient throughthe coating is such as to maximize initial saline corrosion andmechanical erosion when the coated article is put into use, while latermaximizing cathodic sacrificial protection as outer layers of thecoating are eroded or corroded away.

Merely as illustrative of the teachings hereof, one such protectivecoating system may be noted as a combination of cadmium and zincsupplied as a diffusion coating over the surface of a ferrous substrateunder such circumstances that approximately the outer onethird of thecoating thickness is rich in cadmium, while the inner two-thirds is richin zinc. in this manner, initial saline corrosion of zinc is minimizedby having the cadmium outer layer, while the cadmium-rich outer layerproduces far less saline corrosion products than does zinc. Althoughcadmium is virtually insoluble in iron (thus minimizing its utility fordirect diffusion coating into an iron substrate), it is readily solublein zinc, and zinc is readily soluble in iron, so that the combinationprovides for an integrated diffusion coating of zinc into the ironsubstrate and then cadmium into the zinc coating.

It is not, however, necessary that the process be carried out in theabove two-stage fashion. For example, satisfactory results were obtainedin accordance herewith in the protective coating of steel articlesfabricated from a ferrous material such as AMS 5616 (characterized inthe Aerospace Material Specifications of the Society of AutomotiveEngineers as steel including about 13 percent chromium, 2 percentnickel, and 3 percent tungsten) by heating the articles to be coated atabout 730F. for about 12 hours embedded in a powder pack including, byweight, about 10 percent metallic zinc powder (-325 mesh), about 10percent cadmium powder (-325 mesh) with a balance of aboutpercenttabular alumina as an inert filler. After such treatment, thearticles, still embedded in the pack, were additionally heated to atemperature of about 830F. for an additional half hour, and then aircooled. Because of the ready solubility of zinc into iron and the lackof solubility of cadmium, the foregoing process results in a protectivecoating with the outer surface primarily cadmium and the inner portionadjacent the coating-substrate interface rich in zinc in a manner whichprovides for a cadmium-zinc protective outer surface and a layerreservoir of zinc adjacent the substrate interface for cathodicsacrificial protection of the ferrous article when ultimate erosion ofthe outer surface of the coating occurs.

Somewhat similar results were obtained in accordance herewith in atwo-stage pack impregnation process by first producing a preliminarycoating of zinc (about 0.8 mil thick and comprising approximately 13percent iron and 87 percent zinc) on the ferrous article to be coated byembedding it in a powdered pack including about percent zinc, avaporizable halogen or halide energizer (0.25 percent ammonium iodideand 0.125 percent iodine), and alumina as an inert filler, and heatingat about 830F. for 12 hours in a closed retort (all in known and wellunderstood manner). Thereafter the thus coated article was embedded in apowdered pack comprising about 3.4 percent zinc powder (-325 mesh), 16.6percent cadmium powder (-325 mesh) and the balance tabular alumina foran additional coating cycle at about 730F. for 12 hours to provide anouter cadmium coating layer integrally and diffusion-bonded withtheferrous article and the preliminary zinc coating.

Although, as noted above, electroplating of the protective coating maynot give optimum results because of a lack of integrity which maydevelop between the electroplated layer and the substrate, suchtechniques are appropriate for the preliminary deposition of variouscomponents of the protective coating in accordance herewith providedthey can be subsequently integrated by diffusion or interdiffusion ofthe various components. For example, satisfactory results have beenachieved in accordance herewith by diffusion coating a zinc layer on aferrous substrate (such as AMS 5616) by the technique noted above(embedding in a pack comprising alumina, 10 percent zinc powder, 0.25percent ammonium iodide and 0.125 percent iodine for a heating cycle of12 hours at 830F.) and thereafter electroplating cadmium over the thuscoated ferrous article from a conventional cyanide electroplating bath.Thereafter, the plated article was heated in an inert powdered pack atabout 650F. for 4 hours to promote diffusion of the plated cadmium layerinto the zinc-coated surface of the ferrous article for ultimatediffusion integration of the several components of the protectivecoating and to provide a cadmium-rich outer surface under which was azinc-rich sacrificially protective layer bonded to the ferroussubstrate.

As will be understood from the disclosure thereof, cadmium-zinccombinations are particularly attractive for multi-component protectivecoatings in accordance herewith because both such metals have high vaporpressures at relatively low temperatures and thus satisfy one of therequirements for direct vapor deposition in a powder pack cementationprocess. Furthermore, the vapor pressures of these two metals areadequately high for satisfactory deposition at treating temperatureswhich do not adversely affect the mechanical properties of the hardenedand tempered steels of the types referred to earlier from which theferrous metal articles are likely to be fabricated for applications towhich this invention is particularly related (e.g., coating temperaturesless than 900F.).

Furthermore, the cadmium-zinc combination is particularly interesting inthis connection because the physical vapor phase deposition of a metal(as opposed to a volatile compound of that metal) in a thermallyhomogenious powdered cementation pack is appropriate if the metaldissolves in the substrate to form an alloy therein the vapor pressureof which at coating temperature is lower than that of the metal beingdeposited. Although cadmium does not bear this relationship with iron,zinc does behave in this manner with respect to the ferrous substrate,and cadmium is soluble enough in zinc to be satisfactorily depositedsimultaneously with zinc into a ferrous substrate and/or into a ferroussubstrate previously coated with zinc.

Additionally, as noted above, these two metals are particularlyattractive regarding the erosion and sacrificial protection of ferrousarticles because zinc is highly cathodically protective of ferroussubstrates (although not itself sufficiently resistant to direct salinecorrosion) and cadmium is highly resistant to saline corrosion (althoughnot itself sufficiently sacrificial to protect fully a ferrous substrateas with zinc, and the cadmium-zinc combination is more resistant tomechanical abrasion than either component alone. Similarly, the relativesolubilities of cadmium and zinc in either zinc or iron automaticallycontrol the concentration gradient through the coating so as toconcentrate cadmium on the outer surface while the inner layer of thecoating adjacent the substrate interface is primarily rich insacrificially protective zinc.

As will be apparent from the foregoing, a diffusion coating having acadmium-rich surface and a zinc-rich undercoat is capable of affordingadequate cathodic protection of the ferrous substrate while, at the sametime, avoiding severe surface corrosion or white rust formation in ahighly saline atmosphere. As the outer (cadmium) surface becomespartially or wholly removed by erosion or abrasion so as to reduce thecoating thickness and even locally expose the substrate, then thezinc-rich undercoat thus exposed will afford increased cathodicsacrificial protection in the weakened areas. Even under suchconditions, white rust deposits from the zinc form only in those areaswhere the zinc undercoat is exposed by erosion, not over the entirecoating surface, and additional protection against saline corrosion andpitting of the ferrous substrate is extended by the cathodic orsacrificial protection of the zinc increasingly at the end of theexpected service life of the coating where such protection is mostcrucial as the thickness of the coating becomes more and more eroded tothe point of ultimate necessary replacement, repair, or recoating of theferrous article.

Cadmium-zinc coatings of the type described above have been tested in asalt spray cabinet in accordance with Federal Test Method No. 811.151ato assess the protectability thereof, and in all such official testing,as well as other experience evaluations, the protectiveness of suchcoatings and the prolonged useful life thereof has far exceeded resultswith other previously acceptable zinc or cadmium or nickel coatingsutilized for similar applications as discussed above.

Considering another multi-component coating system with whichsatisfactory results are achieved in accordance herewith, one may note atechnique involving first coating the ferrous substrate with a diffusioncoating of zinc, thereafter electroplating a thin layer of nickel on tothe zinc-coated surface, and thereafter additionally applying adiffusion coating of zinc with interdiffusion of all three layers fordiffusion-bonding to the ferrous substrate and for driving the outerzinc layer into and through the nickel layer so as to provide an outercoated surface rich in nickel (for initial corrosion and abrasionresistance) and an inner layer rich in zinc for sacrificial protectionupon eroding of the outer layer. In accordance with this embodiment, theferrous article is first coated by impregnation in a zinccontainingcementation pack, as noted above, for 12 hours at 830 F., after which athin layer of nickel is electroplated on to the thus coated surface byany of known means such as utilizing a conventional nickel sulphamatebath employing a current density of about 0.4 amps per square inch and aplating time of about 4 minutes. Thereafter the plated article is againembedded in the zinc cementation powdered pack for an additional 2 hoursheating at about 830 F.

The above noted treatment cycles are particularly designed to facilitateinterdiffusion and bonding between all layers of the coating and betweenthe zinc undercoat and the substrate. In this way, an efficientmetallurgical bond is assured at all compositional interfaces, therebyreducing the possibility of layer spalling in use. The coating producedin accordance with this embodiment has a composition profile or aconcentration gradient through the coating such that the zinc content ofthe coating drops from about 60 percent at a point approximatelyone-third of the way into the coating, and then rises to a constantlevel of about 88 percent over the inner one-third of the coating. Thenickel content, on the other hand, rises steadily from the surface toreach a maximum (corresponding to the zinc minimum) at about one-thirdthe thickness of the coating, and thereafter decreases toward thesubstratecoating interface at which point it approximates whatever isthe original nickel content (if any) of the base metal being coated.

Although a substantially pure zinc diffusion coating was applied as thelast step to the outer surface of the above example, the diffusionconditions of the application of the zinc layer are such as to cause thezinc to diffuse into and through the nickel layer, thus uniting theelectroplated nickel and decreasing the surface content of zinc. Forthis reason, the third step involves a diffusion coating of zinc for nomore than about 2 hours, because a heavier application of zinc wouldresult in too high zinc concentration on the outer surface and, thus,have little or no advantage over merely a zinc coating over the entirearticle. lndeed, for many applications, the first zinc coating step maybe eliminated, with satisfactory results in accordance herewith havingbeen achieved by directly electroplating nickel on the ferrous article,and then applying an outer zinc diffusion coating from a cementationpack with diffusion of the zinc into and through the electroplated layerto bond it to the surface of the ferrous article but still maintainingthe cencentration gradient of high nickel content at the surface andhigh zinc content for sacrificial protection adjacent thecoating-substrate interface. Satisfactory results have been achieved byapplying the zinc diffusion coating over the electroplated nickel from apack such as noted above at a temperature of 750-850 F. for about 12hours. The presence of highly sacrificial zinc between the outer nickellayer and the ferrous substrate eliminates the above-noted difficultieswith cadmium-nickel systems whereby erosion of the coating and exposureof adjacent areas of nickel and ferrous substrate actually increaseselectrochemical corrosion because of the fact that nickel is noble withrespect to ferrous metals in saline atmospheres and, consequently, notcathodically sacrifically protective thereof.

A further illustrative of multi-metal coating systems embodying and forpractising this invention may be noted the diffusion coating obtainedwith satisfactory results on AMS 6304 steel utilizing a combination ofzinc and lead. Such coatings were produced from a single-stagecementation process using a powdered pack including, by weight, 5percent lead, 5 percent zinc, percent tabular alumina, and 0.5 percentammonium iodide, in which pack the AMS 6304 articles were heated atabout 900 F. for 20-30 hours. The resulting coating provided a highconcentration of lead at the outer surface, with a preponderance of zincand very little lead in the undercoating adjacent the substrate surface.Here again, the surface concentration of lead decreases the immediatesaline corrosion of zinc (with the undesirable white rust" formationcharacteristic of such corrosion), while the internal concentration ofzinc affords enhanced sacrificial protection when the outer surface ofthe coating begins to show the effect of mechanical erosion or abrasion.

In saline atmospheres, lead is considerably less anodic than zinc, asdesired in accordance herewith, and relatively insoluble in the iron ofthe ferrous substrate, but soluble in the alloy produced between theferrous substrate and zinc.

As will be apparent from the foregoing examples, satisfactory resultshave been achieved with coatings on steel materials and using thezinc-cadmium, zincnickel, or zinc-lead systems as disclosed, byproducing the coatings at treating temperatures no higher than about 900F. or less. With the types of steel-base articles noted above, as iswell understood, a significant change or deterioration in metallurgicaland mechanical properties (particularly fatigue life, tensile strength,and thermal shock resistance) may occur with metal articles fabricatedfrom such materials if the articles are subjected to a post-fabricationtemperature treatment above 900-1,000 F. When the continued maintenanceof high mechanical properties is an important consideration in thearticle being coated (as with the jet aircraft engine compressorcomponents referred to above), it is of some significance to be able toprovide whatever post-fabrication surface coating may be desired forcorrosion protection at coating or treating temperatures below those atwhich the base metal article itself undergoes undesired crystallographicor metallurgical transformations, and it is in this area, among others,where the corrosion-resistant coatings according to this invention areparticularly advantageous, in addition to the above-noted quality ofproviding preliminary corrosion and erosion protection at the surfacewith increased cathodic sacrificial protection as the coating isinevitably worn away.

It has been possible to devise diffusion coating systems andaccelerators therefor which permit the diffusion coating of metal suchas aluminum at coating temperatures far below those conventionallyrequired for aluminum-base diffusion coatings (as disclosed in copendingapplication Docket No. 24-669, filed of even date herewith) to avoid theabove noted metallurgical difficulty, and some of such systems (notablythose combining aluminum and zinc) may be considered as having anelectrochemical potential in saline atmospheres which would becathodically sacrificially protective of ferrous substrates. Similarly,as disclosed in co-pending application Docket No. 24-670, filed of evendate herewith, other aluminum-base diffusion coatings are provided toprovide cathodic protection of ferrous substrates in saline atmospheresand at relatively low coating temperatures. In some of such other typesof coatings, moreover, there is also to be found the additional featurehere whereby the outer surface of the coating is rich in metalpreliminarily resistant to saline corrosion and mechanical erosion,while inner layers of the coating are rich in a metal providing cathodicprotection of the substrate although, perhaps, itself deficient indirect or initial corrosion resistance in the saline atmosphere againstwhich protection is desired.

For example, utilizing a pack comprising 20 percent aluminum, 1 percenteach cadmium and zinc, 0.5 percent ammonium iodide, and 0.25 percenturea, with the balance being tabular alumina, AMS 6304 articles werecoated at 900F. during a30 hour heating cycle to produce coatings ofabout 1 mil in thickness and having the composition of about 1-10percent zinc and 60-65 percent alumimum at the substrate-coatinginterface, the balance being iron. The zinc content decreased to zeroaway from the substrate-coating interface, while the aluminum contentincreased toward the surface, thus producing the desired concentrationgradient in accordance herewith whereby initial protection of the outersurface from both corrosion and erosion is attributableto the aluminum,while a zinc concentration near the substrate produces a potential forcathodic protection as the thickness of the coating is eroded orcorroded away in use. Nevertheless, as noted below, the total potentialdifference between such coatings and substrate may not be as great aswith the zinccadmium coatings in accordance herewith, but, with certainsubstrate materials, the aluminum-zinc coatings are sufficiently anodicto give the desired cathodic protection as the coating is worn thinnerin use.

Also, as will be apparent from the foregoing, the coating systemsembodying and for practising this invention'utilize the combination of ahighly sacrificial metal of greater solubility in the substrate and aless sacrificial metal of less solubility in the substrate, and,preferably, also greater resistance to preliminary corrosion anderosion, with, of course, the further admonition that the two metalsmust be compatible with each other in the coating environment andsusceptible of being diffusion coated as desired. It may also be notedthat the more sacrificial of the two elements should possess anelectrochemical potential with respect to the substrate sufficientlyhigh to provide the desired cathodic protection thereof in salineatmospheres, but, preferably, no higher than is practically necessary todo That is, if the more sacrificial coating metal is unnecessarilyhighly anodic with respect to the substrate, the principal result willbe that it will be sacrificed, when electrochemical corrosion commences,at an unnecessarily high rate (however much it may protect thesubstrate), thus ultimately decreasing the total service life of thewhole protective coating. As illustrative of the foregoing, one may notesome comparative electrode potential data regarding a number of theforegoing illustrative examples.

The electrode potentials of a number of samples were measured in a 3percent NaCl solution at 22 C. with reference to a standard saturatedcalomel electrode (E 0.242 volts on the hydrogen scale) after fiveminutes of immersion in the salt solution, with the specimens beingcylinders 1 inch long and 7/16 inch in diameter. As will be understood,the more anodic surface was indicated by a higher negative electrodepotential, which indicates a greater tendency for the surface metal todissolve in or be corroded by the saline environment or atmosphere.Merely as illustrative of data obtained in such determinations, it maybe noted that uncoated specimens of AMS 5616 and 6304 showed potentialsof, respectively, 0.29 v. and 0.57 v., with such substantial variationsapparently being attributable to the fact that the higher proportion ofchromium in AMS 5616 substantially lowers the negative potential.

An AMS 5616 specimen with a diffusion coating of zinc alone showed apotential of 0.92 v., thus confirming the tremendous propensity for thezinc to combine with the saline atmosphere, however cathodicallyprotective the zinc might be for the ferrous substrate. It was foundthat such high negative potential of the zinc coating could besubstantially reduced (e.g., down to 0.83 v.) by chemical conversion ofthe diffused coating by immersion of the coated specimens in a bath ofzinc phosphate in known manner, but the results of such conversion,particularly in atmopsheres producing excessive erosion or mechanicalabrasion, may be short lived. A specimen coated with a zinc-cadmiumcoating in accordance herewith (produced by the single-stage packcementation process noted above) gave a potential of -0.81 v.

Similarly, regarding AMS 6304 specimens, a diffusion coating of Zincalone had a potential of -0.9l v., which was reduced to 0.82 v. bychemical conversion in a zinc phosphate bath. A zinc-cadmium diffusioncoating in accordance herewith and produced by a single-stage packcementation process had potential of -0.8l v. Additional specimens ofAMS 5616 and 6304 coated with the aluminum-zinc diffusion coating notedabove had potentials of, respectively, *078 v. and 0.77 v., which werefurther somewhat reduced by chemical conversion of the coated surface inaluminum chromate solutions (such as Alodine) to give, respectively,-0.74 v. and 0.72 v.

As will be apparent from the foregoing data, all the various coatingsindicated are considerably more anodic than the ferrous substrate, thusoffering sacrifical protection. Nevertheless, the pure zinc coatingsexhibit such high negative potential that corrosion thereof in salineatmospheres would be excessively rapid, even if there were not theadditional disadvantage of the excessive formation of white rustdeposits on the article. Even reducing this negative potential byadditional chemical treatment of the coated article produces noparticular advantage over the zinc-cadmium coatings in accordanceherewith, lacks the permanence thereof due to the interdiffusion of bothmetals, and requires an additional treatment step, while lacking theconcentration gradient through the coating which provides for erosionand corrosion resistance at the coated surface and increased sacrificialprotection nearer the substrate as the coating thickness becomes erodedaway.

It has been developed in accordance herewith that adequate sacrificialprotection of the substrate is obtained if the negative potential of theprotective coating is at least 0.2 -0.3 v. higher than that of thesubstrate. Such a variance is preferred for applications to which thisinvention particularly relates because, with local erosion of thecoating and/or pinholes or cracks therein, the exposed area ofsacrificial metal, as compared to the exposed area of substrate, may notbe expected to be large enough to offer satisfactory cathodic protectionif the potential difference between the two metals is less than about0.2 v. As noted above, however, it is not preferred to have thispotential difference substantially above the minimum because, in thatevent, the principal result is merely to accelerate the rate of loss ofthe protective metal by corrosion without increasing the amount ofsacrificial protection afforded. Generally, and as a practical matter,satisfactory results have been achieved in accordance herewith if thepotential difference between the substrate and the protective coating iswithin the range of about 0.3-0.7 v. for coating thicknesses within thel2 mil range.

Considering the foregoing, the above electrode potential data indicatethat, whereas the aluminum-zinc coating noted above might givesubstantial sacrificial protection on AMS 5616 steel articles, it fallsfar below the protection afforded by the zinc-cadmium coatings on AMS6304 articles, undoubtedly because of the much lower proportion of zincin the coating and notwithstanding the fact that most of the zinc isconcentrated toward the substrate. Thus, although the aluminum-zinccoatings may offer enhanced overall protection against corrosion anderosion in saline atmospheres, such protection must largely beattributable to coating integrity or resistance other than cathodicprotection as contrasted with the zinc-cadmium coatings where totalprotection is achieved first by the inherent resistance of thecadmium-rich surface and, thereafter, by sacrificial protection of thezinc-rich in terior of the coating as the coating thickness becomeseroded or corroded away.

As will be apparent from the foregoing, there are provided in accordanceherewith coating materials and techniques for providing multi-componentdiffusion coatings on metal articles by simple, even one-step packcementation procedures for protection of the metal article from erosiveand corrosive saline environments. The coatings embodying and forpractising this invention are readily applied at temperatures low enoughso that the coating step does not induce crystallographic ormetallurgical transformations in the article being coated which mightaffect the mechanical properties thereof.

Furthermore, these multi-component coatings are produced on the basemetal article in such fashion that the coatings become more anodictoward the substratecoating interface for increased cathodic sacrificialprotection of the substrate as the surface of the coatings becomeseroded away. Thus, the surface of the coating is rich in a component ofincreased corrosion and erosion resistance, while the inner portions ofthe coatings are rich in a component offering greater sacrificialprotection of the substrate although that component may haveconsiderably less. corrosion or erosion resistance. In any event, ifthere are components of the coating which are not anodic with respect tothe substrate, electrochemical contact between the substrate and suchless noble components is avoided despite the interdiffusion of allcomponents and the substrate for complete unification and integrationthereof against a possiblity of fractures or spalling or separation ofcoating and substrate even under intense mechanical strain and thermalshock conditions.

While the methods and compositions set forth above form preferredembodiments of this invention, this invention is not limited to theseprecise methods and compositions, and changes may be made thereinwithout departing from the scope of this invention which is defined inthe appended claims.

What is claimed is:

1. A method of providing a ferrous metal article with a multi-componentsurface diffusion coating for both mechanical and cathodic protectionagainst erosion by abrasion and corrosion by saline environments whichcomprises,

coating said article with a first coating metal of zinc which is bothdiffusably soluble in the surface of said article and substantiallyanodic electrochemically with respect thereto when diffused into thesurface thereof and a second coating metal selected from the groupconsisting of cadmium, nickel and lead which is both less anodic thanzinc and more resistant than zinc to said saline environment,

and diffusing both said coating metals thereon embedded in a powderedcoating pack at a temperature ranging up to about 900F. to effectmetallurgical integration and bonding of said coating metals and saidarticle forming said multi-component surface diffusion coating on saidarticle with said zinc being concentrated adjacent the article-coatinginterface and said second coating selected from the group cadmium,nickel and lead being concentrated adjacent the outer surface of saidmulticomponent coating.

2. The method of claim 1, wherein said first and second coating metalsare applied by embedding said article in a pack containing said firstand second coating metals dispersed through an inert filler, anddiffusion coating said metals from said pack into the surface of saidarticle such that the first coating metal is concentrated adjacent themetal substrate and said second coating metal is concentrated adjacentthe outer surface of said multi-component coating.

3. The method of claim 1, wherein said second coating metal is cadmium.

4. The method of claim 1, wherein the second coating metal is lead.

5. The method of claim 1, wherein the second coating metal is nickel.

2. The method of claim 1, wherein said first and second coating metalsare applied by embedding said article in a pack containing said firstand second coating metals dispersed through an inert filler, anddiffusion coating said metals from said pack into the surface of saidarticle such that the first coating metal is concentrated adjacent themetal substrate and said second coating metal is concentrated adjacentthe outer surface of said multi-component coating.
 3. The method ofclaim 1, wherein said second coating metal is cadmium.
 4. The method ofclaim 1, wherein the second coating metal is lead.
 5. The method ofclaim 1, wherein the second coating metal is nickel.